This study aims to descrbe the distribution within the primate spinal cord of the non-myelinated and small myelinated dorsal root fibers. Many of these small fibers convey pain and temperature information; larger, myelinated dorsal root fibers mediate touch, muscle and position sense. Pain and temperature information is conveyed to the thalamus and brain stem by the spinothalamic and spinoreticular tracts. Determining the sites of termination of the primary afferents and their relation to spinothalamic neurons is an essential step in defining the neuronal chain of the pain and temperature sensory pathways. At the root entry zone, the large fibers enter the dorsal columns, and also send collateral axons into the dorsal horn. The small fibers enter Lissauer's tract, sending branches into the dorsal horn. Two problems arise in tracing the small fibers within the dorsal horn: a) to distinguish the small afferent fibers from the axon collaterals of the larger afferents of the dorsal columns, and b) to distinguish the small afferent fibers from intrinsic axons of substantia gelatinosa neurons that also travel in Lissauer's tract and re-enter the dorsal horn. The only published anatomical study of lesions restricted to Lissauer's tract compared degeneration of this tract with that following dorsal rhizotomy (LaMotte, 1976). Lissauer's tract was found to contain 2 populations of small afferents, distinguished by their different distributions to the dorsal horn and different rates of degeneration. The first stage of this proposal is to extend these findings by identifying, with the electron microscope, the degenerating synaptic terminals of these 2 afferent populations and their synaptic relationships with dorsal horn neurons. Secondly, since the normal morphology of these terminals is obscured by degenerative changes, the synaptic configuration of normal afferent will be analyzed: they will be identified by labelling dorsal root ganglion cells with horseradish peroxidase or tritiated amino acids. Based on these 2 descriptions, labelling and degeneration methods can be combined to further distinguish the classes of small afferents, the small intrinsic fibers and collaterals of large afferents.